You cannot specify an alt tag for the cover image as EasyBlog will automatically use the title of your blog post as the alt attribute for the post cover. If you want to customize this, you could modify the file /components/com_easyblog/themes/wireframe/blogs/latest/post.cover.php

ok thank you, so are you saying that the cover image will be crawled by SEO standards or not?
eventually how shall I modify that file? Im not a coder, what about the other images I can add to a post, where do I write ALT for those?

You can use JCE editor to add ALT tag in image :
- Download from https://www.joomlacontenteditor.net/downloads/editor/pro
- Install from your Joomla backend installer page
- Switch to use JCE editor from backend > Easyblog > settings > general > composer > screenshot : http://take.ms/Ctxsy
- Start a blog post > http://take.ms/idBfQ

OR you can download my attached file and replace into this file location ->JoomlaFolder/templates/YourCurrentTemplate/html/com_easyblog/blogs/entry/post.cover.php (you have to manually create those new folder)

When you add post cover from your blog post, it will auto add the image alt tag into your blog post for this post cover, but this image alt it will always show that image name.